Biomechanics of C-7 transfacet screw fixation

Laboratory investigation

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Object

The small diameter of the pedicle can make C-7 pedicle screw insertion dangerous. Although transfacet screws have been studied biomechanically when used in pinning joints, they have not been well studied when used as part of a C7–T1 screw/rod construct. The authors therefore compared C7–T1 fixation using a C-7 transfacet screw/T-1 pedicle screw construct with a construct composed of pedicle screws at both levels.

Methods

Each rigid posterior screw/rod construct was placed in 7 human cadaveric C6–T2 specimens (14 total). Specimens were tested in normal condition, after 2-column instability, and once fixated. Nondestructive, nonconstraining pure moments (maximum 1.5 Nm) were applied to induce flexion, extension, lateral bending, and axial rotation while recording 3D motion optoelectronically. The entire construct was then loaded to failure by dorsal linear force.

Results

There was no significant difference in angular range of motion between the 2 instrumented groups during any loading mode (p > 0.11, nonpaired t-tests). Both constructs reduced motion to < 2° in any direction and allowed significantly less motion than in the normal condition. The C-7 facet screw/T-1 pedicle screw construct allowed a small but significantly greater lax zone than the pedicle screw/rod construct during lateral bending, and it failed under significantly less load than the pedicle screw/rod construct (p < 0.001).

Conclusions

When C-7 transfacet screws are connected to T-1 pedicle screws, they provide equivalent stability of constructs formed by pedicle screws at both levels. Although less resistant to failure, the transfacet screw construct should be a viable alternative in patients with healthy bone.

Abbreviations used in this paper: ROM = range of motion; LZ = lax zone; SZ = stiff zone.

Article Information

Address correspondence to: Neil R. Crawford, Ph.D., Neuroscience Publications, 350 West Thomas Road, Phoenix, Arizona 85013. email: neuropub@chw.edu.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Schematics showing (A) lateral and (B) posterior views of the C-7 transfacet screw trajectory. Reproduced with permission from Horn et al.: J Neurosurg Spine 9:200–206, 2008. Figure provided courtesy of Indiana University.

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    Configurations for flexibility and load-to-failure tests. Left: In the flexibility test, adjustable pulleys were used in conjunction with a standard servohydraulic test frame, enabling 2 equal and opposite forces separated by a small distance (pure moment) to be applied through a looped string when the piston advanced upward. Optical markers on each level are visible for tracking vertebral motion. Right: In the failure test, a posterior force was applied to the entire hardware construct, using a cable with looped ends in conjunction with the servohydraulic test frame. Each cable end was routed around both the C-7 and T-1 screws under the rods so that both rostral and caudal screws were loaded equivalently. The piston of the servohydraulic frame pulled the cable posteriorly while a load cell in line with the piston measured applied load.

  • View in gallery

    Bar graphs showing mean angular motion for the normal (A), destabilized (B), and instrumented (C) conditions of the specimens. Columns represent ROM. On each column the horizontal line denotes the division between the LZ and SZ. Error bars show SDs of the ROM.

  • View in gallery

    Bar graph demonstrating mean ultimate strength for the entire construct (C7–T1) in specimens with C-7 transfacet screws compared with specimens with C-7 pedicle screws. In both groups, the C-7 screw head was interconnected bilaterally by longitudinal rods to pedicle screws at T-1. Error bars show SDs.

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